The present invention relates in general to reaction injection molding of a foam padding layer between a rigid substrate and a skin covering layer for automotive interior trim panels, and, more specifically, to a method of de-gassing during formation of the foam layer.
A common type of padded or “soft” interior trim panel for transportation vehicles can be made using a foam-in-place manufacturing technique. The trim panel is a composite article comprising a rigid substrate carrying a foam layer and covered by a flexible skin. The substrate and skin are separately made and then loaded into a reaction-injection molding (RIM) tool where a reactive mixture which starts out as a liquid is injected into a space between the substrate and skin. The constituents of the reactive mixture react to form the foam, which is allowed to cure and the composite article is then removed from the RIM tool.
Foam-in-place panels have been used for door panels and instrument panels in automobiles, for example. An instrument panel may include a rigid substrate that is injection molded with a shape that provides a desired contour and support for mounting various accessories. For example, a passenger air bag system may be assembled onto the substrate behind a seamless (i.e., hidden) door that forms or is attached as part of the substrate. During deployment, the air bag forces open the door which tears through the foam and skin to allow the air bag to enter the passenger compartment.
Conventional foam-in-place articles have been subject to problems in which the foam layer insufficiently adheres to the substrate. This problem is especially acute in an instrument panel having a steel air bag door forming part of the substrate. Due to differences in adherence between the foam layer and the air bag door as compared to surrounding plastic areas of the substrate, a smooth and even surface of the skin is not obtained. Instead, imperfections such as waviness or other “read-through” of the door onto the outer surface can be seen. Moreover, when the foam lacks sufficient adherence to the air bag door, pieces of the foam may become loose during deployment of the air bag when the foam and skin are torn by the door. The presence of flying debris during a vehicle crash is undesirable.
A typical foam is comprised of a polyurethane produced by mixing polyol and isocyanate. This reaction also produces carbon dioxide gas as a byproduct inside the RIM tool. The RIM tool acts as a closed mold which must be de-gassed before final curing of the article in order to avoid bubbles forming under the skin. The conventional solution for de-gassing has been to drop the RIM tool out of clamp (i.e., opening the RIM tool at least slightly in order to depressurize the interior mold cavity). However, the need for de-gassing occurs prior to full curing of the foam. This can result in less than full adherence of the foam to the substrate. In particular, a steel air bag door may not yet have been sufficiently heated by the reactive mixture to obtain good foam adherence to the door. Some improvement has been obtained in the prior art by preheating the steel air bag door prior to placing it in the RIM tool. However, such a heating step is undesirable and does not always achieve the desired results.